1. Field of the Invention
The present invention relates to a DC-DC converter.
2. Description of the Related Art
FIG. 4 is a drawing of the circuit of a conventional forward DC-DC converter. In the drawing, reference numeral 2 denotes a transformer having a primary winding 2a, a secondary winding 2b and a reset winding 2c, a DC power source 1 being connected to the primary winding 2a of the transformer 2 through a switching element 3. Reference numeral 4 denotes a reset diode which is connected to the reset winding 2c of the transformer 2 for the purpose of resetting the transformer 2 excited. Reference numerals 6, 7 each denote a rectifying diode connected to the secondary winding 2b; reference numeral 8, a smoothing choke coil; reference numeral 9, a smoothing capacitor; reference numeral 10, a load; and reference numerals 21, 22, 23, a resistance, a diode and a capacitor, respectively, which form a snubber circuit 24.
The operation of the converter will be described below. The switching element 3 is periodically turned on and off by a control circuit (not shown). The energy of the electric power of the DC power source 1 is transmitted forward from the primary winding 2a of the transformer 2 to the secondary winding 2b thereof during the time the switching element 3 is turned on. On the secondary side, the energy is rectified by the rectifying diodes 6, 7, smoothed by the smoothing choke coil 8 and the smoothing capacitor 9 and then supplied to the load 10.
In this sort of converter, the core of the transformer 2, which is excited during the time the switching element 3 is turned on, must be reset at the time the switching element 3 during turned off. In FIG. 4, therefore, the energy is returned to the DC power source 1 through the diode 4 during the time the switching element 3 is turned off.
In addition, an increase in the switching frequency or an increase in the operational speed of the switching element 3 is accompanied by the need for the snubber circuit 24. The snubber circuit 24 is provided for the purpose of securing a safe operating region for the switching element 3 during the switching operation and preventing any loss of the switching element 3.
FIG. 5 is a drawing of waveforms which shows the relation between the drain current I.sub.D and the source/drain voltage V.sub.SD of the switching element 3 in the conventional converter in which character E denotes the voltage applied to the transformer 2. As shown in FIG. 5, at the time T.sub.1 the switching element 3 is turned on, the leakage inductance of the transformer 2 prevents rapid rising of the drain current ID and causes gradual rising of the drain current I.sub.D. At the time T.sub.2 the switching element 3 is turned off, the diode 22 and the capacitor 23 prevent rapid rising of the source/drain voltage V.sub.SD and causes gradual rising of the source/drain voltage V.sub.SD.
In this way, the rise time of the drain current I.sub.D and the rise time of the source/drain voltage V.sub.SD are set to times longer than the rise time t.sub.on of the switching element 3 and the fall time t.sub.off thereof, respectively, so that the switching loss can be reduced.
Most of the energy stored in the leakage inductance of the transformer 2 at the time the switching element 3 is turned on is moved to the capacitor 23 when the element 3 is turned off. However, part of the energy is consumed by the resistance 21. Thus, the loss caused by the resistance 21 increases as the switching frequency of the switching element 3 increases, resulting in a reduction in the conversion efficiency. Since the energy stored in the leakage inductance of the transformer 2 is also increased as the load 10 is increased, the rise time of the source/drain voltage V.sub.SD is changed by the change in the load 10, and the loss caused by the resistance 21 is also increased.